1. Field of the Invention
The present invention relates to a system for controlling the speed changes of an automatic transmission and, more particularly, to a control system for an automatic transmission including a first speed change unit for executing a clutch-to-clutch shift by applying/releasing two frictional engagement elements simultaneously and a second speed change unit for executing a speed change by a one-way clutch.
2. Related Art
In the prior art, there has been widely adopted an automatic transmission which is equipped with a gear speed change mechanism and a plurality of frictional engagement elements for switching torque transmission lines of the gear speed change mechanism. In the automatic transmission thus constructed, there may occur the clutch-to-clutch shift in which a predetermined speed change is executed by applying/releasing two frictional engagement elements simultaneously. In this clutch-to-clutch shift, a shift shock by a tie-up of the frictional engagement elements or an over-shoot of the engine may occur in dependence upon the propriety of the apply/release timings of the frictional engagement elements participating in the speed change. This makes it necessary to control the oil pressure for controlling the apply/release timings of the individual frictional engagement elements, in accordance with the progress of the shift. One example of this speed change control system is disclosed in JPA-4-228967.
The control system, as disclosed, is constructed to control the oil pressure of the frictional engagement elements participating in the clutch-to-clutch shift. The oil pressure of the applied side frictional engagement element is kept at a low level while the oil pressure of the released side frictional engagement element being reduced, and the oil pressures of these frictional engagement elements are then raised. At the same time, the slip of the applied side frictional engagement element is measured, and this slip state is controlled by controlling the oil pressure of the released side frictional engagement element. Finally, the applied side frictional engagement element is applied by raising its oil pressure.
As described above, the clutch-to-clutch shift has to be controlled while causing the oil pressures of the frictional engagement elements participating in the shift to relate to each other. Therefore, the oil pressures are controlled by detecting the slip state of the frictional engagement elements or by monitoring the RPM of the output shaft. However, the oil pressure being directly controlled and the oil pressure in the frictional engagement elements cannot be completely equalized. In addition, the viscosity of the oil changes with the temperature, and the coefficients of friction of the frictional engagement elements will change with time and have individual differences so that the torque capacities of the individual frictional engagement elements fail to shoot the targets. This may cause the shock due to the tie-up of the frictional engagement elements or the over-shoot of the engine.